1. Technical Field
The present invention relates to a control device for an all-wheel-drive vehicle.
2. Related Art
All-wheel-drive (AWD) (or four-wheel-drive (4WD)) vehicles that exhibit good traveling performance on steep slope roads, bumpy roads, slippery roads (e.g., snowy and muddy roads), etc. have been widely used. As for AWD vehicles, a full-time system and a part-time system are known. In the part-time system, two-wheel drive and four-wheel drive are switched between when needed. The full-time system implements four-wheel drive all the time by disposing a center differential between the front and rear wheels and allowing for a revolution speed difference between the front and rear wheels. On the other hand, the part-time system includes, for example, main drive wheels directly coupled to the engine and driven wheels (sub driving wheels) coupled to the engine through a transfer clutch. The part-time system controls the coupling force of the transfer clutch in accordance with conditions such as a road surface condition and a traveling condition so as to adjust distribution of a driving force to the driven wheels and to switch between two-wheel drive and four-wheel drive.
Japanese Examined Patent Application Publication (JP-B) No. 8-29681 discloses a 4WD-vehicle control method for reducing rattling noise of a driveline (rattling noise due to abutting of gear teeth) produced when reversal of torque (reversal of torque from a reverse drive state to a drive state) occurs in response to shifting of an accelerator-pedal off state to an accelerator-pedal on state. More specifically, according to this control method, in the case where a distribution ratio of torque to the front and rear wheels is adjusted by changing an engaging force of a friction engagement device, if a throttle opening is less than or equal to a predetermined opening, control is performed to increase the engaging force of the friction engagement device so that the driving states of the front and rear wheels approach the directly coupled state.
According to this control method, since the friction engagement device such as a torque distribution control clutch engages to produce a lockup state in response to a decrease in the throttle opening, rattling noise of the driveline is successfully reduced even if the accelerator-pedal off state is shifted to the accelerator-pedal on state and consequently reversal of torque occurs while the vehicle is traveling.
As described above, according to the 4WD-vehicle control method disclosed in JP-B No. 8-29681, it is possible to reduce rattling noise of the driveline (rattling noise due to abutting of gear teeth) produced when reversal of torque (reversal of torque from the reverse drive state to the drive state) occurs in response to shifting of the accelerator-pedal off state to the accelerator-pedal on state. However, if the engaging force of the friction engagement device (transfer clutch) is increased in a state where the accelerator pedal is released (in the accelerator-pedal off state), abnormal noise (rattling noise) may be produced by gears of the driveline depending on the traveling state of the vehicle.
It is desirable to provide a control device for an all-wheel-drive vehicle including a transfer clutch that adjusts a driving force transferred from an automatic transmission to a driven-wheel-side driveline, the control device being capable of reducing abnormal noise (rattling noise) that may be produced by gears of the driveline in a state where an accelerator pedal is released (in an accelerator-pedal off state).
After careful examinations of the issue described above, the inventors have obtained the following knowledge. In a state where the accelerator pedal is released (in an off state) and toque applied to a gear of a driveline (for example, a secondary reduction drive/driven gear) is substantially zero (that is, the backlash of the gear is reduced toward neither the drive side nor the coast side), if an electromagnetic valve (solenoid valve) for adjusting hydraulic pressure is driven to increase the coupling force of the transfer clutch because a difference in revolution speed occurs between the front wheels and the rear wheels in response to steering on a steering wheel, transfer torque of the transfer clutch fluctuates due to hydraulic pressure fluctuation (hydraulic vibration) based on the driving frequency of the electromagnetic valve. As a result, the gear of the driveline (for example, the secondary reduction drive/driven gear) vibrates, producing gear rattling noise (abnormal noise).